Acoustic wave device with trench portions and narrow interdigital transducer tip portions for transverse mode suppression

ABSTRACT

An acoustic wave device, a radio frequency filter and an electronics module are provided. The acoustic wave device, comprises a layer of piezoelectric material, a pair of interdigital transducer electrodes disposed on an upper surface of the layer of piezoelectric material, each interdigital transducer electrode including a bus bar and a plurality of electrode fingers extending from the bus bar through a central region of the interdigital transducer electrode towards an edge region of the interdigital transducer electrode, each of the plurality of electrode fingers having a width in a direction perpendicular to the extension of the electrode fingers that is smaller in the edge regions than in the central regions, and trench portions located in the upper surface of the layer of piezoelectric material, the trench portions overlapping with the edge regions of the interdigital transducer electrodes. The acoustic wave device provides effective suppression of transverse modes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Patent Application Ser. No. 63/362,680, titled “ACOUSTICWAVE DEVICE WITH TRENCH PORTIONS AND NARROW INTERDIGITAL TRANSDUCER TIPPORTIONS FOR TRANSVERSE MODE SUPPRESSION,” filed Apr. 8, 2022, to U.S.Provisional Patent Application Ser. No. 63/362,679, titled “ACOUSTICWAVE DEVICE WITH TRENCH PORTIONS FOR TRANSVERSE MODE SUPPRESSION,” filedApr. 8, 2022, and to U.S. Provisional Patent Application Ser. No.63/362,682, titled “METHOD OF MANUFACTURE OF ACOUSTIC WAVE DEVICE WITHTRENCH PORTIONS FOR TRANSVERSE MODE SUPPRESSION,” filed Apr. 8, 2022,the entire content of each is incorporated herein by reference for allpurposes.

BACKGROUND Field

Aspects and embodiments disclosed herein relate to an acoustic wavedevice, a method of manufacture of the same and a radio frequency filterand electronic module including the same. In particular, embodiments ofthe invention relate to an acoustic wave device including trenchportions in a piezoelectric layer and narrow interdigital transducer tipportions for transverse mode suppression.

Description of the Related Technology

Multilayer piezoelectric substrates (MPSs) are often used in acousticwave devices, such as surface acoustic wave (SAW) devices. Severalstructures for suppressing unwanted transverse modes in such devices areknown. However, the various known structures each have differentdrawbacks.

FIGS. 1A and 1B show one type of acoustic wave device 100. FIG. 1A is across-section through the line marked A on the plan view of FIG. 1B. Theacoustic wave device 100 has a multilayer piezoelectric substrate (MPS)including a carrier substrate 102, a layer of dielectric material 104disposed on an upper surface of the carrier substrate 102, and a layerof piezoelectric material 106 disposed on the layer of dielectricmaterial 104. An interdigital transducer electrode (IDT) 108 is disposedon top of the layer of piezoelectric material 106. In the acoustic wavedevice 100 of FIGS. 1A and 1B, the electrode fingers in the IDT 108include hammer head portions 110 to suppress the transverse modes. Thehammer head portions 110 are sections of the electrode fingers in edgeregions E of the IDT that have a width (in a direction perpendicular tothe lengthwise extension of the electrode fingers) larger than the widthof each finger in a central region C of the IDT 108. A duty factor (DF)of the IDT 108 is greater in the edge regions E of the IDT as comparedto the duty factor of the IDT in the central region C of the IDT.

In general, the width of the IDT fingers compared to the width of thespacing between the IDT fingers sets the duty factor (DF). Specifically,the duty factor is defined as the fraction of the IDT width spanned bythe width of the IDT fingers (in the direction of propagation of themain surface acoustic wave to be generated). Increasing the width of theIDT fingers, while maintaining the position of the center of each IDTfinger, increases the duty factor.

The hammer head portions 110 of the device of FIGS. 1A and 1B reduce theacoustic velocity in the edge regions E compared to the central regionC. This velocity reduction creates a piston mode distribution to reducetransverse modes. To obtain a large enough velocity difference fortransverse mode suppression through a larger DF in the edge regions E,the DF of the central region C of the IDT should be less than 0.5. Thisis because the velocity of the main acoustic mode changes rapidly withDF when the DF is less than 0.5, compared to when DF is greater than 0.5and the velocity of the main acoustic mode does not vary with DF asmuch. The use of a DF narrower than 0.5 in the central region C leads toa decrease in the static capacitance. A smaller static capacitance leadsto a larger size device for a given impedance, as static capacitancesets the limit on the IDT size. Therefore, the hammer head structure ofFIGS. 1A and 1B can lead to an undesirable increase in size of theacoustic wave device 100.

FIGS. 2A and 2B show another type of acoustic wave device 200. Theacoustic wave device 200 of FIGS. 2A and 2B is identical to that ofFIGS. 1A and 1B, except that the acoustic wave device 200 does notinclude the hammer head portions 110. Instead, the acoustic wave device200 includes a passivation layer 210 of silicon nitride (SiN) disposedthe IDT 208 that is thickest in the central region C of the IDT 208. Thepassivation layer 210 creates a piston mode distribution to suppress thetransverse modes in a similar fashion to the acoustic wave device 100 ofFIGS. 1A and 1B. However, a drawback of the thicker passivation layer210 in the central region is that the electromechanical couplingcoefficient (K2) is reduced.

FIGS. 3A and 3B show another similar acoustic wave device 300. Insteadof the hammer head portions 110 or passivation layer 210, the acousticwave device 300 includes a pair of mass loading strips 310 disposed overthe edge regions E of the IDT 308. The mass loading strips are strips ofa heavy dielectric material, or strips of heavy conductive material ifsuitably isolated, for example, by the thin passivation layer 312 shownin FIG. 3A. The mass loading strips 310 also suppress the transversemodes, however the central region C of the IDT can often become damagedduring formation of the mass loading strips 310 on the edge regions E.

FIGS. 4A and 4B show another similar acoustic wave device 400. Theacoustic wave device 400 of FIGS. 4A and 4B is identical to that ofFIGS. 1A and 1B, except that the acoustic wave device 400 does notinclude the hammer head portions 110. Instead, thicker portions 410 areincluded, which are portions where the thickness of the IDT 408 in adirection perpendicular to the plane of the layer of piezoelectricmaterial 406 has been increased in the edge regions E of the IDT 408.Such a configuration again establishes a piston mode distribution.However, formation of thicker portions 410 is difficult to achieve.

SUMMARY

According to one embodiment there is provided an acoustic wave device.The acoustic wave device comprises a layer of piezoelectric material, apair of interdigital transducer electrodes disposed on an upper surfaceof the layer of piezoelectric material, each interdigital transducerelectrode including a bus bar and a plurality of electrode fingersextending from the bus bar through a central region of the interdigitaltransducer electrode towards an edge region of the interdigitaltransducer electrode, each of the plurality of electrode fingers havinga width in a direction perpendicular to the extension of the electrodefingers that is smaller in the edge regions than in the central regions,and trench portions located in the upper surface of the layer ofpiezoelectric material, the trench portions overlapping with the edgeregions of the interdigital transducer electrodes.

In one example the widths of the plurality of electrode fingers of eachinterdigital transducer electrode are smaller in both the edge region ofthat interdigital transducer electrode and the edge region of the otherinterdigital transducer electrode.

In one example a duty factor of the pair of interdigital transducerelectrodes in the edge regions of the interdigital transducer electrodesis less than a duty factor of the pair of interdigital transducerelectrodes in the central regions.

In one example the duty factor of the pair of interdigital transducerelectrodes in the edge regions is about 0.4, and the duty factor of thepair of interdigital transducer electrodes in the central region isabout 0.5.

In one example the electrode fingers of each interdigital transducerelectrode interleave with one another in an active region of the pair ofinterdigital transducer electrodes, and form gap regions between theends of the fingers of one of the interdigital transducer electrodes andthe bus bar of the other interdigital transducer electrode.

In one example the edge regions of the pair of interdigital transducerelectrodes are located within the active region and on opposing sides ofthe active region.

In one example the edge regions are located at the distal ends of theelectrode fingers.

In one example the active region includes the central regions and theedge regions of the interdigital transducer electrodes, each edge regionextending from the tips of the plurality of electrode fingers of one ofthe interdigital transducer electrodes towards the center of the centralregion.

In one example the trench portions in the upper surface of the layer ofpiezoelectric material also overlap with at least part of the gapregions.

In one example each of the interdigital transducer electrodes includes asecond bus bar that is located within the gap region.

In one example the plurality of electrode fingers have lesser widths inregions between the bus bar and second bus bar of each interdigitaltransducer electrode.

In one example, the acoustic wave device further comprises dummyelectrodes extending from the second bus bar partially through the gapregion toward an adjacent edge region.

In one example the trench portions are located in the areas of the uppersurface of the layer of piezoelectric material that are overlapped bythe edge regions of the interdigital transducer electrodes and are notcovered by the interdigital transducer electrodes.

In one example the trench portions extend discontinuously in thedirection of propagation of an acoustic wave to be generated by the pairof interdigital transducer electrodes.

In one example the acoustic wave device further comprises a layer ofdielectric material, the layer dielectric material having an uppersurface disposed against a lower surface of the layer of piezoelectricmaterial.

In one example the acoustic wave device further comprises a carriersubstrate, the carrier substrate including a second layer of one ofaluminum nitride, silicon nitride, polysilicon, or amorphous silicon anupper surface disposed against a lower surface of the layer ofdielectric material and a third layer of one of silicon, siliconcarbide, sapphire, diamond-like carbon or quartz having an upper surfacedisposed against a layer surface of the second layer.

In one example the layer of piezoelectric material is formed of amaterial selected from the group consisting of lithium tantalate,aluminum nitride, lithium niobate, or potassium niobate.

In one example the layer of dielectric material includes silicondioxide, or doped silicon material.

In one example the carrier substrate is formed of a material selectedfrom the group consisting of silicon, aluminum nitride, silicon nitride,magnesium oxide spinel, magnesium oxide crystal, quartz, diamond,diamond-like carbon, or sapphire.

In one example each interdigital transducer electrode is formed from asingle layer of etch resistant material.

In one example the etch resistant material is selected from the groupconsisting of copper, platinum, tungsten, molybdenum, ruthenium,iridium, gold, and silver.

In one example each interdigital transducer electrode is formed from oneor more lower layers of material and an upper layer of etch resistantmaterial.

In one example the upper layers have lesser maximum widths than thelower layers.

In one example the upper layers have trapezoidal cross-sections.

In one example the etch resistant material is selected from the groupconsisting of copper, platinum, tungsten, molybdenum, ruthenium,iridium, gold, and silver.

In one example each interdigital transducer electrode includes a masklayer on the upper surface of the interdigital transducer electrode.

In one example the mask layer is a layer of chromium.

In one example the acoustic wave device further comprises a protectivelayer disposed over the upper surfaces of the pair of interdigitaltransducer electrodes and the layer of piezoelectric material.

In one example the protective layer is formed from one or more of thegroup consisting of silicon nitride, silicon oxynitride, and silicondioxide.

According to another embodiment there is provided a radio frequencyfilter comprising at least one acoustic wave device. The acoustic wavedevice includes a layer of piezoelectric material, a pair ofinterdigital transducer electrodes disposed on an upper surface of thelayer of piezoelectric material, each interdigital transducer electrodeincluding a bus bar and a plurality of electrode fingers extending fromthe bus bar through a central region of the interdigital transducerelectrode towards an edge region of the interdigital transducerelectrode, each of the plurality of electrode fingers having a width ina direction perpendicular to the extension of the electrode fingers thatis smaller in the edge regions than in the central regions, and trenchportions located in the upper surface of the layer of piezoelectricmaterial, the trench portions overlapping with the edge regions of theinterdigital transducer electrodes.

According to another embodiment there is provided an electronics modulecomprising at least one radio frequency filter that includes at leastone acoustic wave device. The at least one acoustic wave device includesa layer of piezoelectric material, a pair of interdigital transducerelectrodes disposed on an upper surface of the layer of piezoelectricmaterial, each interdigital transducer electrode including a bus bar anda plurality of electrode fingers extending from the bus bar through acentral region of the interdigital transducer electrode towards an edgeregion of the interdigital transducer electrode, each of the pluralityof electrode fingers having a width in a direction perpendicular to theextension of the electrode fingers that is smaller in the edge regionsthan in the central regions, and trench portions located in the uppersurface of the layer of piezoelectric material, the trench portionsoverlapping with the edge regions of the interdigital transducerelectrodes.

Still other aspects, embodiments, and advantages of these exemplaryaspects and embodiments are discussed in detail below. Embodimentsdisclosed herein may be combined with other embodiments in any mannerconsistent with at least one of the principles disclosed herein, andreferences to “an embodiment,” “some embodiments,” “an alternateembodiment,” “various embodiments,” “one embodiment” or the like are notnecessarily mutually exclusive and are intended to indicate that aparticular feature, structure, or characteristic described may beincluded in at least one embodiment. The appearances of such termsherein are not necessarily all referring to the same embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of at least one embodiment are discussed below withreference to the accompanying figures, which are not intended to bedrawn to scale. The figures are included to provide illustration and afurther understanding of the various aspects and embodiments, and areincorporated in and constitute a part of this specification, but are notintended as a definition of the limits of the invention. In the figures,each identical or nearly identical component that is illustrated invarious figures is represented by a like numeral. For purposes ofclarity, not every component may be labeled in every figure. In thefigures:

FIG. 1A is a cross-sectional side view of an acoustic wave device of theprior art;

FIG. 1B is a plan view of the acoustic wave device of FIG. 1A;

FIG. 2A is a cross-sectional side view of an acoustic wave device of theprior art;

FIG. 2B is a plan view of the acoustic wave device of FIG. 2A;

FIG. 3A is a cross-sectional side view of an acoustic wave device of theprior art;

FIG. 3B is a plan view of the acoustic wave device of FIG. 3A;

FIG. 4A is a cross-sectional side view of an acoustic wave device of theprior art;

FIG. 4B is a plan view of the acoustic wave device of FIG. 4B;

FIG. 5A is a plan view of an acoustic wave device according to aspectsof the present disclosure;

FIG. 5B is a cross-sectional view of the acoustic wave device of FIG.5A;

FIG. 5C is a cross-sectional view of the acoustic wave device of FIG.5A;

FIG. 5D is a cross-sectional view of the acoustic wave device of FIG.5A;

FIG. 5E is a cross-sectional view of the acoustic wave device of FIG.5A;

FIG. 6A is a graph showing a comparison of admittance curves of anacoustic wave device according to aspects disclosed herein and anacoustic wave device without trench portions;

FIG. 6B is a graph showing a comparison of admittance curves of anacoustic wave device according to aspects disclosed herein with anacoustic wave device without trench portions;

FIG. 6C is a graph showing a comparison of quality factor curves of anacoustic wave device according to aspects disclosed herein with anacoustic wave device without trench portions;

FIG. 7A is a graph showing a comparison of admittance curves of anacoustic wave device according to aspects disclosed herein and anacoustic wave device according to FIG. 4A;

FIG. 7B is a graph showing a comparison of admittance curves of anacoustic wave device according to aspects disclosed herein with anacoustic wave device according to FIG. 4A;

FIG. 7C is a graph showing a comparison of quality factor curves of anacoustic wave device according to aspects disclosed herein with anacoustic wave device according to FIG. 4A;

FIG. 8A is a graph showing a comparison of admittance curves of acousticwave devices according to aspects disclosed herein;

FIG. 8B is a graph showing a comparison of admittance curves of acousticwave devices according to aspects disclosed herein;

FIG. 8C is a graph showing a comparison of quality factor curves ofacoustic wave devices according to aspects disclosed herein;

FIG. 9A is a graph showing a comparison of admittance curves of acousticwave devices according to aspects disclosed herein;

FIG. 9B is a graph showing a comparison of admittance curves of acousticwave devices according to aspects disclosed herein;

FIG. 9C is a graph showing a comparison of quality factor curves ofacoustic wave devices according to aspects disclosed herein;

FIG. 10A is a plan view of an acoustic wave device according to aspectsdisclosed herein;

FIG. 10B is a cross-sectional view of the acoustic wave device of FIG.10A;

FIG. 10C is a cross-sectional view of the acoustic wave device of FIG.10A;

FIG. 11A is a graph showing a comparison of admittance curves ofacoustic wave devices according to aspects disclosed herein;

FIG. 11B is a graph showing a comparison of admittance curves ofacoustic wave devices according to aspects disclosed herein;

FIG. 11C is a graph showing a comparison of quality factor curves ofacoustic wave devices according to aspects disclosed herein;

FIG. 12A is a plan view of an acoustic wave device according to aspectsdisclosed herein;

FIG. 12B is a cross-sectional view of the acoustic wave device of FIG.12A;

FIG. 12C is a cross-sectional view of the acoustic wave device of FIG.12A;

FIG. 12D is a cross-sectional view of the acoustic wave device of FIG.12A;

FIG. 12E is a cross-sectional view of the acoustic wave device of FIG.12A;

FIG. 13A is a graph showing a comparison of admittance curves ofacoustic wave devices according to aspects disclosed herein;

FIG. 13B is a graph showing a comparison of admittance curves ofacoustic wave devices according to aspects disclosed herein;

FIG. 13C is a graph showing a comparison of quality factor curves ofacoustic wave devices according to aspects disclosed herein;

FIG. 14A is a plan view of an acoustic wave device according to aspectsdisclosed herein;

FIG. 14B is a plan view of an acoustic wave device according to aspectsdisclosed herein;

FIG. 15A is a graph showing a comparison of admittance curves ofacoustic wave devices according to aspects disclosed herein;

FIG. 15B is a graph showing a comparison of admittance curves ofacoustic wave devices according to aspects disclosed herein;

FIG. 15C is a graph showing a comparison of quality factor curves ofacoustic wave devices according to aspects disclosed herein;

FIG. 16A shows a step of manufacturing an acoustic wave device accordingto aspects disclosed herein;

FIG. 16B shows a step of manufacturing an acoustic wave device accordingto aspects disclosed herein;

FIG. 16C shows a step of manufacturing an acoustic wave device accordingto aspects disclosed herein;

FIG. 16D shows a step of manufacturing an acoustic wave device accordingto aspects disclosed herein;

FIG. 16E shows a step of manufacturing an acoustic wave device accordingto aspects disclosed herein;

FIG. 17A is a plan view of an acoustic wave device according to aspectsdisclosed herein;

FIG. 17B is a cross-sectional view of the acoustic wave device of FIG.17A;

FIG. 17C is a cross-sectional view of the acoustic wave device of FIG.17A;

FIG. 17D is a cross-sectional view of the acoustic wave device of FIG.17A;

FIG. 17E is a cross-sectional view of the acoustic wave device of FIG.17A;

FIG. 18A is a plan view of an acoustic wave device according to aspectsdisclosed herein;

FIG. 18B is a cross-sectional view of the acoustic wave device of FIG.18A;

FIG. 18C is a cross-sectional view of the acoustic wave device of FIG.18A;

FIG. 18D is a cross-sectional view of the acoustic wave device of FIG.18A;

FIG. 18E is a cross-sectional view of the acoustic wave device of FIG.18A;

FIG. 19A is a plan view of an acoustic wave device according to aspectsdisclosed herein;

FIG. 19B is a cross-sectional view of the acoustic wave device of FIG.19A;

FIG. 19C is a cross-sectional view of the acoustic wave device of FIG.19A;

FIG. 19D is a cross-sectional view of the acoustic wave device of FIG.19A;

FIG. 19E is a cross-sectional view of the acoustic wave device of FIG.19A;

FIG. 20 shows an example of a ladder filter in which multiple acousticwave devices according to aspects disclosed herein may be combined;

FIG. 21 is a block diagram of one example of a filter module that caninclude one or more acoustic wave devices according to aspects disclosedherein;

FIG. 22 is a block diagram of one example of a front-end module that caninclude one or more filter modules including acoustic wave devicesaccording to aspects of the present disclosure; and

FIG. 23 is a block diagram of one example of a wireless device includingthe front-end module of FIG. 22 .

DETAILED DESCRIPTION

This application relates to an acoustic wave device, a radio frequencyfilter, and an electronics module. The acoustic wave device comprises alayer of piezoelectric material, a pair of interdigital transducerelectrodes disposed on an upper surface of the layer of piezoelectricmaterial, each interdigital transducer electrode including a bus bar anda plurality of electrode fingers extending from the bus bar towards anedge region of the interdigital transducer electrode at the distal endsof the electrode fingers, and trench portions located in the uppersurface of the layer of piezoelectric material, the trench portionsoverlapping with the edge regions of the interdigital transducerelectrodes. The acoustic wave device provides effective suppression oftransverse modes.

It is to be appreciated that embodiments of the methods and apparatusesdiscussed herein are not limited in application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the accompanying drawings. Themethods and apparatuses are capable of implementation in otherembodiments and of being practiced or of being carried out in variousways. Examples of specific implementations are provided herein forillustrative purposes only and are not intended to be limiting. Also,the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use herein of“including,” “comprising,” “having,” “containing,” “involving,” andvariations thereof is meant to encompass the items listed thereafter andequivalents thereof as well as additional items. References to “or” maybe construed as inclusive so that any terms described using “or” mayindicate any of a single, more than one, and all of the described terms.

Aspects and embodiments of the present disclosure are described belowthrough embodiments of acoustic wave devices, in particular surfaceacoustic wave (SAW) devices. However, as would be understood by theskilled person, various different excitation modes are possible inacoustic wave filters and devices, particularly MPS devices. As well assurface acoustic waves other types of acoustic wave are possible such asboundary acoustic waves and guided acoustic waves. References to surfaceacoustic waves and surface acoustic wave (SAW) devices in the followingdescription are not intended to limit the disclosure from including orcovering other possible types of acoustic waves and acoustic wavedevices.

FIG. 5A is a plan view of a surface acoustic wave (SAW) device accordingto a first embodiment. FIGS. 5B and 5C show cross-sections through thelines in FIG. 5A labeled A and B respectively. FIGS. 5D and 5E showpartial cross-sections through the lines in FIG. 5A labeled X and Yrespectively (with only two IDT fingers shown in FIGS. 5D and 5E forclarity).

The acoustic wave device 500 includes a carrier substrate 502, a layerof dielectric material 504 disposed on an upper surface of the carriersubstrate 502, and a layer of piezoelectric material 506 disposed abovethe layer of dielectric material 504 on the upper surface of the carriersubstrate 502. Together the carrier substrate 502, layer of dielectricmaterial 504, and layer of piezoelectric material 506 may be referred toas a multilayer piezoelectric substrate (MPS).

Any piezoelectric material may be used as the layer of piezoelectricmaterial 506, for example, including but not limited to lithiumtantalate (LiTaO₃), aluminum nitride (AlN), lithium niobate (LiNbO₃), orpotassium niobate (KNbO₃). Various materials may also be used in thelayer of dielectric material 504 and in the carrier substrate 502. Oneexample of a material that may be utilized for the layer of dielectricmaterial 504 is silicon dioxide (SiO₂). Other examples may include dopedmaterials such as F doped SiO₂, or Ti doped SiO₂. One example of amaterial that may be utilized for the carrier substrate 502 is silicon(Si), however aluminum nitride, silicon nitride, magnesium oxide spinel,magnesium oxide crystal, quartz, diamond, DLC (diamond-like carbon) andsapphire may all also be used as the carrier substrate.

The carrier substrate 502 may be formed of a material having a lowercoefficient of linear expansion and/or a higher thermal conductivityand/or a higher toughness or mechanical strength than the piezoelectricmaterial. The carrier substrate 502 may both increase the mechanicalrobustness of the piezoelectric material during fabrication of the SAWdevice and increase manufacturing yield, as well as reducing the amountby which operating parameters of the SAW device change with temperatureduring operation. The carrier substrate 502 may be referred to as a highimpedance support substrate.

An interdigital transducer (IDT) 508 is disposed on top of the layer ofpiezoelectric material 506 and is configured to generate a surfaceacoustic wave in the multilayer piezoelectric substrate. In use, the IDT508 excites a main acoustic wave having a wavelength λ along a surfaceof the multilayer piezoelectric substrate. The acoustic wave isconcentrated in the top two layers (the layer of dielectric material 504and layer of piezoelectric material 506). The carrier substrate 502 (inthis case silicon) may have a high impedance meaning the acoustic waveis reflected at the boundary between the carrier substrate 502 and thelayer of dielectric material 504, confining the surface acoustic wave inthe top two layers. In some embodiments, the thickness of the layer ofdielectric material 504 may be between 0.1λ and 1λ, preferably between0.1λ and 0.5λ, and the thickness of the layer of piezoelectric material506 may be between 0.1λ and 1λ, preferably between 0.1λ and 0.5λ. It isto be understood that the dimensions above are only examples and may beset at different values in different embodiments of acoustic wavedevices to achieve different design goals.

Any type of IDT may be used as the IDT 508 in the acoustic wave device500. For example, a typical IDT will include a pair of interlocking combshaped IDT electrodes. Each electrode of the IDT typically includes abus bar and a plurality of electrode fingers that extend perpendicularlyfrom the bus bar. Typically, the distance between the central point ofeach adjacent electrode finger extending from the same bus bar is equalto the wavelength λ of the surface acoustic wave generated. The bus barsof each of the pair or IDT electrodes are parallel and opposing eachother, and the plurality of electrode fingers of each IDT electrodeextend towards to the bus bar of the opposing electrode, such that theelectrode fingers interlock, typically with a distance of λ/2 betweenthe centers of each set of adjacent electrode fingers extending fromopposite bus bars. The main surface acoustic wave generated by the IDTtravels perpendicular to the lengthwise direction of the IDT electrodefingers, and parallel to the lengthwise direction of the IDT bus bars.

Regardless of the type of IDT used, the IDT 508 has an active regiondefined as the region that the fingers of each interdigital transducerelectrode interleave with one another. The surface acoustic wave isgenerated in the active region of the IDT. The active region of the IDTincludes a central region and two edge regions. The central region islabeled by the letter C in FIG. 5A and the edge regions are labeled bythe letter E. Each edge region E extends from the tips of the pluralityof fingers of one of the electrodes towards the center of the centralregion C. The edge regions E include end portions of the IDT electrodefingers, and the central region C is sandwiched between the edgeregions. The purpose of the edge regions will be discussed in moredetail below. The IDT also includes gap regions labeled by the letter Gin FIG. 5A. The gap regions are located between the ends of the fingersof one of the electrodes and the bus bar of the other electrode. Theregions containing the bus bars are labeled B in FIG. 5A. The dashedlines in FIG. 5A show the boundaries between the above describedregions.

In the embodiment of FIG. 5A, the IDT electrodes 508 each include asecond bus bar 512 that is located within the gap region G. The secondbus bars 512 extend parallel to the bus bars, and are located adjacentto the edge regions E of the IDT 508. The second bus bars 512 arethinner than the bus bars, and may be referred to as “mini bus bars”.The mini bus bars result in the transverse modes being suppressed moreeffectively However, in some embodiments these mini bus bars may beomitted (see the discussion relating to FIGS. 10A to 11C below).

In the embodiments of FIGS. 5A to 5E a double layer IDT 508 is used,with an upper IDT layer 508 a and a lower IDT layer 508 b. Howeversingle layer IDTs may also be used. In general, various IDT structuresare possible, as would be understood by the skilled person, for exampledouble electrode IDTs, or IDTs with dummy electrode fingers may be used.Specific IDT configurations will be discussed in more detail later,taking into consideration the method of manufacture of the device. Moredetail on specific materials that the IDT 508 may be formed of will alsobe discussed later.

The acoustic wave device 500 further includes trench structures in thelayer of piezoelectric material for suppressing the transverse modes.Trench portions 510 are located in the upper surface of the layer ofpiezoelectric material. The trench portions 510 overlap with the edgeregions E of the IDT electrodes 508. The trench portions 510 are locatedwithin the active region of the IDT 508, in the edge regions E of theIDT 508, and form a boundary of the active region running parallel withthe bus bars. The trench portions 510 slow down the acoustic velocity atedge of the active region to set up piston mode distribution, and thussuppress the transverse modes.

As can be seen from FIG. 5A, the trench portions 510 extend parallel tothe bus bars, in the direction of propagation of the main acoustic wavegenerated by the IDT 508. However, the trench portions are only presentin the sections of the upper surface of the layer of piezoelectricmaterial 506 that are overlapped by the edge regions E of the IDT 508and are not covered by the material of the IDT 508. The trench portions510 are only cut into the surface of the layer of piezoelectric material506 that is exposed after the IDT 508 has been formed on the layer ofpiezoelectric material 506. The trench portions 510 are not cut into thesections of the layer of piezoelectric material 506 covered by the IDT508, meaning the trench portions 510 do not run underneath the IDT 508.The layer of piezoelectric material 506 remains at full thicknessunderneath the IDT 508. This is best seen in FIG. 5E, showing the trenchportions 510 cut into the upper surface of the layer of piezoelectricmaterial 506 not covered by the IDT 508, and not cut into the uppersurface of the layer of piezoelectric material 506 covered by the IDT508. A comparison of the cross-sectional views of FIGS. 5B and 5C alsoshows this. Therefore, the trench portions 510 can be described asextending discontinuously in the direction of propagation of the mainacoustic wave generated by the IDT 508 (along the line marked Y in FIG.5A).

The trench portions 510 can be formed in this way by etching thepiezoelectric substrate. In particular, the trenches portions 510 may beetched after the formation of the IDT 508 on the upper surface of thelayer of piezoelectric material 506, with the IDT preventing etching ofthe layer of piezoelectric material 506 underneath the IDT. The methodof manufacture of the device will be discussed in more detail later, inrelation to FIGS. 16A to 18E.

In some embodiments, the trench portions may each have a width (marked win FIG. 5C) in a direction perpendicular to the direction of propagationof an acoustic wave to be generated by the IDT 508 of between about 0.5λand 1λ, where λ is the wavelength of the main acoustic wave to begenerated by the IDT 508. In some embodiments, the trench portions mayeach have a depth (marked h in FIG. 5C) relative to the upper surface ofthe layer of piezoelectric material of between about 0.004λ and 0.02λ,where λ is the wavelength of the main acoustic wave to be generated bythe IDT 508. The effect of the variation of the width w and depth h willbe discussed in more detail in relation to FIGS. 8A to 9C.

As best seen in FIG. 5E, due to the trench portions 510 the sections ofthe electrode fingers of the IDT 508 in the edge regions E (the tips ofthe electrode fingers) are positioned higher relative to the surface ofthe layer of piezoelectric material 506 at the bottom of the trenchportions. Therefore, the trench portions 510 increase the effectivethickness of the IDT electrodes 508 seen by the acoustic wave within theedge regions E. The increased effective thickness of the IDT 508 in theedge regions E results in the piston mode distribution, which suppressesthe transverse modes.

The acoustic wave device 500 has a number of advantages compared to theprior art devices of FIGS. 1A to 4B. Firstly, due to the trench portions510 the piston mode distribution can be implemented without a decreasein the duty factor of the of the central region C of the IDT 508,preventing an unwanted decrease in static capacitance and thus increasein size of the device. Specifically, in the acoustic wave device 500 ofFIGS. 5A to 5E, a DF of 0.5 is used in the IDT 508, which is good forsize reduction. In addition, as discussed in more detail below, thetrench portions 510 can be formed through etching, resulting in easierfabrication in which the central region C is less easily damaged.Lastly, the combination of the trench portions 510 with the MPS resultsin a device with a high Q-factor, high electromechanical couplingcoefficient (K2), excellent temperature coefficient of frequency (TCF),and high power durability.

FIGS. 6A to 6C are simulation graphs showing a comparison betweenadmittance curves (complex FIG. 6A, and real FIG. 6B) and quality factorcurves (Q-factor, FIG. 6C) of an acoustic wave device as disclosedherein with trench portions and a comparative example without trenchportions. In particular, the graphs of FIGS. 6A to 6C include a solidline trace showing the simulation results for the acoustic wave device500 of FIGS. 5A to 5E with a width w of the trench portions 510 equal to1λ and a depth of 0.007λ, where λ is the wavelength of the main acousticwave to be generated by the IDT 508. The dashed line trace is for thecomparative example, which is identical to the acoustic wave device 500of the solid line trace except that is does not include the trenchportions.

As can be seen from the graphs of FIGS. 6A and 6B, many transverse modesare present in the dashed line trace of the comparative example. In thesolid line trace for the acoustic wave device 500 with trench portions510, on the other hand, the transverse modes are greatly suppressed.

FIGS. 7A to 7C show a second comparison simulation study. The graphs ofFIGS. 7A to 7C are the same as FIGS. 6A to 6C, except that in FIGS. 7Ato 7C the prior art acoustic wave device 400 of FIGS. 4A and 4B is usedas the comparative example (the dashed line trace). The simulation ofthe acoustic wave device 400 is based on molybdenum thicker portions 410with a width of 1λ (in the direction of the extension of the electrodefingers) and a height of 0.003λ. As can be seen from the graphs, theperformance of the acoustic wave device 500 of FIGS. 5A to 5E issimilar, but slightly improved compared to the comparative example. Animprovement can be seen in the Q-factor graph of FIG. 7C in particular.

The effect of variation of width w and depth h of the trench portions510 will now be discussed in relation to FIGS. 8A to 9C.

FIGS. 8A to 8C are simulation graphs showing a comparison betweenadmittance curves (complex FIG. 8A, and real FIG. 8B) and quality factorcurves (Q-factor, FIG. 8C) of acoustic wave devices disclosed herein. InFIGS. 8A to 8C, the dotted line trace is simulated for the acoustic wavedevice 500 of FIGS. 5A to 5E with w=1λ and h=0.004λ, the solid linetrace for the acoustic wave device 500 with w=1λ and h=0.007λ, and thedashed line trace for the acoustic wave device 500 with w=1λ andh=0.010λ, where λ is the wavelength of the main acoustic wave to begenerated by the IDT 508. FIGS. 8A to 8C therefore show the effect ofvarying the depth h of the trench portions 510 of the acoustic wavedevice 500 of FIGS. 5A to 5E with a width fixed at 1λ.

As can be seen from FIGS. 8A to 8C, with trench portions 510 with adepth of h=0.004λ there is only a small amount of suppression of thetransverse modes. A depth of h=0.007λ results in a more effectivesuppression of the transverse modes. A depth of h=0.010λ results in evenmore suppression of the transverse modes, however the additional depthof h=0.010λ also begins to cause a split in the main resonance peak.Therefore, for a width of w=1λ, trench portions with a depth h ofbetween about 0.004λ and 0.010λ is preferred for transverse modesuppression.

FIGS. 9A to 9C show similar simulation graphs, using widths of thetrench portions 510 of w=0.5λ for each trace, and depths of h=0.010λ forthe dotted line trace, h=0.015λ for the solid line trace, and h=0.020λfor the dashed line trace, where λ is the wavelength of the mainacoustic wave to be generated by the IDT 508. FIGS. 9A to 9C thereforeshow the effect of varying the depth h of the acoustic wave device 500of FIGS. 5A to 5E with a width fixed at 0.5λ. For a width of w=0.5λ,trench portions with a depth h of between about 0.010λ and 0.020λ ispreferred for transverse mode suppression.

As can be seen from a comparison of FIGS. 8C and 9C, the Q-factor islarger, and therefore improved, when a width of w=1λ is used for thetrench portions 510, compared to w=0.5λ. Therefore, a width w=1λ ispreferred.

FIGS. 10A to 10C show an acoustic wave device 1000 in anotherembodiment. The acoustic wave device 1000 includes a carrier substrate1002, a layer of dielectric material 1004, a layer of piezoelectricmaterial 1006, an IDT 1008 with an upper layer 1008 a and a lower layer1008 b, and trench portions 1010 located in the upper surface of thelayer of piezoelectric material. The acoustic wave device 1000 isidentical to the acoustic wave device 500 of FIGS. 5A to 5E except thatthe IDT 1008 of the acoustic wave device 1000 does not include the minibus bars (second bus bars 512). The cross-sectional views equivalent tothose in FIGS. 5D and 5E would be identical for the acoustic wave device1000 and have therefore been omitted.

FIGS. 11A to 11C are simulation graphs showing a comparison betweenadmittance curves (complex FIG. 11A, and real FIG. 11B) and qualityfactor curves (Q-factor, FIG. 11C) of acoustic wave devices disclosedherein. In FIGS. 11A to 11C the dashed line trace shows the simulationfor the acoustic wave device 500 of FIGS. 5A to 5E, and the solid linetrace shows the simulation for the acoustic wave device 1000 of FIGS.10A to 10C. Both traces are based on simulations for devices with w=1λand h=0.007λ for the trench portions 510 and 1010, where λ is thewavelength of the main acoustic wave to be generated by the IDT.

As highlighted in the dashed circles in FIGS. 11A and 11B, thesuppression of the transverse modes is improved when the acoustic wavedevice includes the mini bus bars 512, compared to when the mini busbars are omitted. Therefore, adding the mini bus bars 512 of FIG. 5Aimproves suppression of higher order transverse modes. Althoughinclusion of the mini bus bars 512 is optional, it is preferable.

FIGS. 12A to 12E show an acoustic wave device 1200 in anotherembodiment. The acoustic wave device 1200 includes a carrier substrate1202, a layer of dielectric material 1204, a layer of piezoelectricmaterial 1206, an IDT 1280 with an upper layer 1208 a and a lower layer1208 b, and trench portions 1210 located in the upper surface of thelayer of piezoelectric material. The acoustic wave device 1200 isidentical to the acoustic wave device 1000 of FIGS. 10A to 10C exceptthat the IDT 1208 of the acoustic wave device 1200 further includesnarrow tip portions 1214 in the IDT fingers. The narrow tip portions1214 in conjunction with the trench portions 1210 suppress thetransverse modes.

In more detail, the narrow tip portions 1214 are sections of each of theplurality of electrode fingers in the IDT electrodes 1208 that have awidth in a direction perpendicular to the extension of the electrodefingers that is smaller in the edge regions E of the IDT electrodes thanin the central regions C of the IDT electrodes. The narrow tip portions1214 are located in the edge region E of each IDT electrode. The distalends of the plurality of electrode fingers in each IDT electrode (theends furthest from the respective bus bar) have a reduced width. Thenarrow tip portions 1214 are also located at the sections of each IDTelectrode that overlap with the edge region E of the other IDTelectrode. The widths of the plurality of electrode fingers of each IDTelectrode are therefore smaller in both the edge region E of that IDTelectrode and the edge region E of the other IDT electrode, as best seenin the view of FIG. 12A.

A duty factor (DF) of the pair of IDT electrodes 1208 in the edgeregions E is less than a duty factor of the pair of interdigitaltransducer electrodes in the central region C, best seen in thecomparison of FIGS. 12D and 12E. As can be seen from FIG. 12E, thetrench portions 1210 that are cut out of the layer of piezoelectricmaterial 1206 extend a greater distance in the direction of propagationof the acoustic wave to be generated by the IDT 1208, due to the largerseparation between the electrode fingers in the edge regions E. As inthe previous embodiments, the trench portions 1210 do not extendunderneath the sections of the plurality of electrode fingers in theedge region E (in this case the narrow tip portions 1214).

The DF of the IDT 1208 in the edge regions E may be about 0.4, and theDF of the IDT 1208 in the central region C may be about 0.5. Asdiscussed earlier, a duty factor of 0.5 in the central region C isbeneficial, as reducing the DF below 0.5 in the central region leads toa decrease in the static capacitance and therefore an increase in thesize of the device.

FIGS. 13A to 13C are simulation graphs showing a comparison betweenadmittance curves (complex FIG. 13A, and real FIG. 13B) and qualityfactor curves (Q-factor, FIG. 13C) of the acoustic wave device 1200 ofFIGS. 12A to 12E, compared to the acoustic wave device 500 of FIGS. 5Ato 5E.

The dashed line trace in FIGS. 13A to 13C shows the simulation for theacoustic wave device 500 of FIGS. 5A to 5E, including the mini bus bars512, and with trench portions 510 having width w=1λ and depth h=0.07λ,where λ is the wavelength of the main acoustic wave to be generated bythe IDT 508. The solid line trace in FIGS. 13A to 13C shows thesimulation for the acoustic wave device 1200 of FIGS. 12A to 12E,without any mini bus bars, but instead with the narrow tip portions 1214with DF=0.4 in the edge regions E and DF=0.5 in the central region C,and with trench portions 1210 having width w=1λ and depth h=0.015λ,where λ is the wavelength of the main acoustic wave to be generated bythe IDT 1208.

As can be seen from FIGS. 13A to 13C, the suppression of the transversemodes is very comparable between the acoustic wave device 1200 withnarrow tip portions 1214, and the acoustic wave device 500 of FIGS. 5Ato 5E, despite the lack of mini bus bars in the acoustic wave device1200 of FIGS. 12A to 12E. The acoustic wave device 1200 including thenarrow tip portions 1214 therefore provides an improvement over theacoustic wave device 1000 of FIGS. 10A to 10C.

The acoustic wave device 1200 of FIGS. 12A to 12E performs comparably tothe acoustic wave device 500 of FIGS. 5A to 5E when the depth h of thetrench portions 1210 is larger than that of the trench portions 510.Here a depth of h=0.015λ combined with the narrow tip portions 1214 iscomparable to a depth of h=0.007λ without the narrow tip portions.

Although described above without mini bus bars, the acoustic wave device1200 of FIGS. 12A to 12E including the narrow tip portions 1214 couldalso include mini bus bars in some embodiments, similar to the mini busbars 512 described in FIGS. 5A to 5E.

FIG. 14A shows a plan view of another embodiment. The acoustic wavedevice 1400 of FIG. 14A is identical to the acoustic wave device 500 ofFIGS. 5A to 5E, except that the widths of the trench portions 1410 havebeen extended into the gap regions G. As well as overlapping with theedge regions E of the IDT 1408, the trench portions 1410 in the uppersurface of the layer of piezoelectric material 1406 also overlap with atleast part of the gap regions G. The other identical components of theacoustic wave device 1400 have been given analogous reference numerals.

In general, the trench portion 1410 could be stretched into the gapregions G, away from the active region of the IDT 1408, by any distance.In the embodiment of FIG. 14A, the acoustic wave device 1400 includesmini bus bars 1412 (second bus bars), and the trench portions extendinto the gap regions G all the way up to the edge of the mini bus bars1412. The trench portions 1410 each have a width w in a directionperpendicular to the direction of propagation of an acoustic wave to begenerated by the IDT 1408 that extends from the respective edge region Eto the second bus bar 1412 of the other IDT electrode.

An alternate embodiment is shown in FIG. 14B, where no mini bus bars arepresent. The acoustic wave device 1400 of FIG. 14B is identical to theacoustic wave device 1000 of FIGS. 10A to 10C, except that the widths ofthe trench portions 1410 have been extended into the gap regions G.Again, the trench portion 1410 could be stretched into the gap regionsG, away from the active region of the IDT 1408, by any distance. In theembodiment shown in FIG. 14B, the trench portions 1410 extend into thegap regions G all the way up to the edge of the main bus bars of the IDT1408. The trench portions 1410 each have a width w in a directionperpendicular to the direction of propagation of an acoustic wave to begenerated by the IDT 1408 that extends from the respective edge region Eto the bus bar (bus bar region B) of the other electrode. As the trenchportions 1410 are not cut into the areas of the layer of piezoelectricmaterial 1406 that are covered by the IDT 1408, the width w of thetrench portions 1410 cannot extend beyond the gap region G into the busbar region B.

In either of the embodiments of FIG. 14A or 14B, the properties of theacoustic wave device are largely unchanged by the width of the trenchportions 1410 extending into the gap region G. However, such aconfiguration is easier to manufacture, as in general wider trenchportions 1410 are easier to fabricate than narrower trench portions.

FIGS. 15A to 15C are simulation graphs showing a comparison betweenadmittance curves (complex FIG. 15A, and real FIG. 15B) and qualityfactor curves (Q-factor, FIG. 15C) of the acoustic wave device 1400 ofFIG. 14A, compared to the acoustic wave device 500 of FIGS. 5A to 5E.

The solid line trace in FIGS. 15A to 15C shows the simulation for theacoustic wave device 1400 of FIGS. 14A, with trench portions 1410 havinga depth h=0.007λ, where λ is the wavelength of the main acoustic wave tobe generated by the IDT 1408, and a width w extended through the entireedge regions E into the gap regions G up to the mini bus bars 1412. Thedashed line trace in FIGS. 15A to 15C shows the simulation for theacoustic wave device 500 of FIGS. 5A to 5E, including the mini bus bars512, and with trench portions 510 having width w=1λ and depth h=0.007λ,where λ is the wavelength of the main acoustic wave to be generated bythe IDT 508.

As can be seen from FIGS. 15A to 15C, the suppression of the transversemodes is very comparable between the acoustic wave device 1400 of FIG.14A and the acoustic wave device 500 of FIGS. 5A to 5E. Therefore, it isacceptable to increase the width of the trench portions 1410 into thegap regions G without degrading performance of the device, thus enablingsimpler manufacture of the device.

In some embodiments, the acoustic wave devices 1400 of FIGS. 14A and 14Bcould also including the narrow tip portions 1214 described in relationto FIGS. 12A to 12E. The narrow tip portions 1214 of FIGS. 12A to 12Emay be combined with the stretched trench portions 1410.

A method of manufacturing an acoustic wave device as disclosed hereinwill now be described with reference to FIGS. 16A to 16E. The methodwill be described in relation to the acoustic wave device 500 of FIGS.5A to 5E. However, the method could be appropriately adapted tomanufacture the other embodiments described herein, as would beunderstood by the skilled person.

In a first step 1602 shown in FIG. 16A a layer of piezoelectric material506 is provided, with a pair of IDT electrodes 508 disposed on an uppersurface of the layer of piezoelectric material 506. In some embodiments,the layer of piezoelectric material 506 may be part of a MPS including alayer of dielectric material and a carrier substrate. In step 1602 anacoustic wave device the same as the acoustic wave device 500 of FIGS.5A to 5E is provided, except that no trench portions are yet present inthe layer of piezoelectric material 506. Various fabrication methods asknown in the art could be used to form the layer of piezoelectricmaterial and IDT of the initial acoustic wave device of step 1602without the trench portions.

In a second step, trench portions 510 are etched into desired locationsin the upper surface of the layer of piezoelectric material 506 notcovered by the IDT 508. The trench portions 510 overlap with the edgeregions E of the interdigital transducer electrodes, as described inFIGS. 5A to 5E. In some embodiments, the trench portions may alsooverlap with at least part of the gap regions G, as described in FIGS.14A and 14B.

In more detail, in some embodiments the etching involves the step 1604shown in FIG. 16B of positioning an etching mask 1650 over the uppersurface of the acoustic wave device. The etching mask is resistant toetching, so that it protects the areas of the upper surface of the layerof piezoelectric material 506 that it covers preventing those areas frombeing affected by the etching process. The areas of the upper surface ofthe layer of piezoelectric material 506 that are not covered by theetching mask 1650 or the IDT 508 remain exposed to the etching process.The etching mask 1650 covers all areas of the layer of piezoelectricmaterial 506 except those where the trench portions 510 are to belocated.

The dashed outline in FIG. 16C shows the position of the etching mask1650 in step 1604 in more detail. As can be seen in FIG. 16C, theetching mask 1650 covers all of the upper surface of the layer ofpiezoelectric material 506 except for in the edge regions E. Therefore,the trench portions 510 will only be formed in the edge regions E duringetching, resulting in an acoustic wave device 500 as shown in FIGS. 5Ato 5E. In other embodiments, a different shape of etching mask 1650 maybe used to form the trench portions of alternative embodiments. Forexample, to form the trench portions 1410 in the embodiments of FIGS.14A and 14B, an etching mask 1650 that leaves some or all of the gapregions G uncovered may be used. The step of positioning the etchresistant mask 1650 includes covering the pair of IDT electrodes and theupper surface of the layer of piezoelectric material other than in theedge regions E and, in some embodiments, at least part of the gapregions G.

Once the etching mask is positioned correctly, etching occurs in step1606, as shown in FIG. 16D. The etching step 1606 forms the trenchportions 510 in the upper surface of the layer of piezoelectric material506 not covered by the etching mask 1605 or the IDT 508. Various typesof etching processes may be used, for example, any of chemical etching,laser etching, dry etching, vapor phase etching, wet etching, or plasmaetching. The etching process can be controlled to set the depth h of thetrench portions 510 cut into the layer of piezoelectric material 506.The etching mask 1650 controls the width w of the trench portions 510cut into the layer of piezoelectric material 506.

In step 1608, shown in FIG. 16E, the etching mask 1650 is removed,leading to the finished acoustic wave device 500 including the trenchportions 510.

The above described etching process reduces the risk of damage to thecentral region C of the IDT compared to, for example, fabrication of themass loading strips of FIGS. 3A and 3B. The IDT 508 including thecentral region is formed before the etching process occurs, and from afabrication point of view, the central region C of the IDT 508 is notdamaged by the etching.

As seen in FIG. 16C, the areas of the acoustic wave device not coveredby the etching mask 1605 during etching also includes sections of theIDT 508. The sections of the IDT 508 that are exposed during etchingneed to be carefully considered to ensure that the etching process doesnot damage the exposed parts of the IDT 508. The specific materials ofthe IDT 508 are chosen to protect the exposed sections of the IDT 508during etching.

For example, in each of the previous described embodiments, a multilayerIDT is used, with an upper IDT layer 508 a,1008 a,1208 a and a lower IDTlayer 508 b,1008 b,1208 b. In embodiments with such IDT configurations,a high density IDT material that is etch resistant is chosen as theupper IDT layer 508 b,1008 b,1208 b. The high density upper IDT layerprotects the exposed sections of the IDT during the etching process,even when not covered by the etching mask 1650. The high density upperIDT layer also protects the surface of the piezoelectric materialunderneath the IDT and therefore the surface of the piezoelectricmaterial underneath the IDT is not removed during the etching process.

The high density IDT material of the upper IDT layer may be any ofcopper Cu, platinum Pt, tungsten W, molybdenum Mo, ruthenium Ru, iridiumJr, gold Au, or silver Ag. Preferably, copper is chosen as the highdensity material for the upper IDT layer 508 b,1008 b,1208 b, as it isresistant to etching chemicals as well as being highly conductive,meaning resistive loss is reduced.

The lower IDT layer 508 a,1008 a,1208 a can include materials that arenot etch resistant, such as aluminum Al, due to the high density upperIDT layer. However, other materials that are etch resistant may still beused as the lower layer in some embodiments, for example a copper Culower layer. In some embodiments, the IDT may include multiple lower IDTlayers underneath the upper IDT layer.

In a specific embodiment, a high density molybdenum Mo layer may be usedas the upper IDT layer 508 b,1008 b,1208 b, and lower density but higherconductivity aluminum Al may be used as the lower layer 508 a,1008a,1208 a.

In general, the IDT may be formed through one or more of mask printing,deposition such as physical vapor deposition, electroplating, a lift-offprocess, a dry etching process, or the like. A lift-off process ispreferred.

In an alternative embodiment, instead of a multilayer IDT as singlelayer IDT may be used. FIGS. 17A to 17E show an acoustic wave device1700 in an embodiment with a single layer IDT 1708. The acoustic wavedevice 1700 includes a carrier substrate 1702, a layer of dielectricmaterial 1704, a layer of piezoelectric material 1706, a single layerIDT 1708 with mini bus bars 1712, and trench portions 1710 located inthe upper surface of the layer of piezoelectric material. The acousticwave device 1700 of FIGS. 17A to 17E is identical to the acoustic wavedevice 500 of FIGS. 5A to 5E, except that a single layer IDT 1708 isused. The single layer IDT could be used in combination with any of theembodiments described previously.

In the acoustic wave device 1700 with a single layer IDT 1708, a highdensity, etch resistant material is used in the IDT 1708, such as thoseidentified above: copper Cu, platinum Pt, tungsten W, molybdenum Mo,ruthenium Ru, iridium Jr, gold Au, or silver Ag. Copper is againpreferable, for the reasons mentioned above.

Either the single layer IDT 1708 or the multilayer IDT 508,1008,1208 maybe combined with one or more thin adhesion layers (not shown), such as alayer of titanium Ti, nickel Ni, or chromium Cr. Such thin adhesionlayers may be located between any of the boundaries between the IDTlayers and piezoelectric layer, which is beneficial for the fabricationprocess.

FIGS. 18A to 18E show another embodiment. The acoustic wave device 1800uses a hard mask layer 1816 to protect the IDT during fabrication,particularly during the etching process. The acoustic wave device 1800includes a carrier substrate 1802, a layer of dielectric material 1804,a layer of piezoelectric material 1806, an IDT 1808 with an upper layer1808 a and a lower layer 1808 b and mini bus bars 1812, and trenchportions 1810 located in the upper surface of the layer of piezoelectricmaterial. The acoustic wave device 1800 is identical to the acousticwave device 500 of FIGS. 5A to 5E, but further includes the mask layer1816.

The mask layer 1816 is applied onto the upper surfaces the IDT 1808before etching to protect the IDT during the etching process. The masklayer 1816 is a thin layer that is not susceptible to etching, andtherefore prevents any damage or etching of the IDT 1808. The mask layer1816 may therefore also be referred to as an etch stop layer. The masklayer 1816 is more difficult to etch than the materials of the IDT 1808.The mask layer 1816 therefore allows any materials to be used in theIDT, without damage occurring to the IDT 1808 during formation of thetrench portions 1810. A material not resistant to etching, for example,aluminum, could be used as the upper surface of the IDT when combinedwith the protective mask layer 1816, without any damage occurring to theIDT.

The IDT when covered by the mask layer 1816 may include any of thefollowing materials: aluminum Al, copper Cu, titanium Ti, platinum Pt,tungsten W, molybdenum Mo, ruthenium Ru, iridium Jr, gold Au, silver Ag,or nickel Ni. The mask layer 1816 may be applied to any of the IDTstructures described above, such as a multilayer IDT as shown in FIGS.18A to 18E, or a single layer IDT, such as the IDT 1708 shown in FIGS.17A to 17E.

The mask layer 1816 may be combined with any of the embodimentsdescribed previously. The mask layer 1816 may be left on after thetrench portions are formed, with the mask layer therefore being presentin the fully fabricated acoustic wave device. This prevents the possiblyof damage to the IDT by trying to remove the mask layer 1816.

Preferably, the mask layer 1816 is a thin layer of chromium Cr. In aspecific embodiment, a chromium layer of between 5 nm and 50 nm is used.Chromium is preferred because it is etch resistant and has a high etchselectivity, meaning it is etched at a slower rate than other materials.(Etch selectivity is the ratio of etch rates between materials.)

In a specific embodiment, a chromium hard mask layer 1816 could be usedin combination with a multilayer IDT with a molybdenum lower layer 1808b, and an etch susceptible aluminum upper layer 1808 a.

The acoustic wave device 1800 of FIGS. 18A to 18E also includes anoptional protective layer 1818 disposed over the entire upper surface ofthe acoustic wave device. The protective layer 1818 covers the uppersurfaces of the IDT 1808 (in this embodiment, the mask layer 1816), andthe upper surfaces of the layer of piezoelectric material 1806 notcovered by the IDT (including within the trench portions 1810). Theprotective layer 1818 is a thin coating covering the whole of the topsurface of the acoustic wave device 1800. The protective layer isdisposed onto the upper surface of the acoustic wave device after theetching step is complete, and the etching mask 1650 has been removed,and is left on the device permanently.

The protective layer 1818 acts as a passivation layer and also protectsthe IDT from any external chemical damage. In some embodiments, theprotective layer 1818 may be formed from one or more of silicon nitride(SiN), silicon oxynitride (SiON), or silicon dioxide (SiO₂). amultilayer combination of these materials may also be used as theprotective layer 1818.

The protective layer 1818 may be included in any of the embodimentsdescribed previously, including acoustic wave devices without the masklayer 1816.

FIGS. 19A-19E show another embodiment. FIG. 19A is a plan view of theSAW device 1900. FIGS. 19B and 19C show cross-sections through the linesin FIG. 19A labeled A and B respectively. FIGS. 19D and 19E show partialcross-sections through the lines in FIG. 19A labeled D and E,respectively (with only two IDT fingers shown in FIGS. 19D and 19E forclarity).

The embodiment of FIGS. 19A-19E is similar to that of FIGS. 5A-5E, butincludes additional features that may be used individually or incombination in any of the embodiments disclosed herein. The acousticwave device 1900 includes a carrier substrate 1902, a layer ofdielectric material 1904, a layer of piezoelectric material 1906, an IDT1980 with an upper layer 1908 a and a lower layer 1908 b, and trenchportions 1910 located in the upper surface of the layer of piezoelectricmaterial. The acoustic wave device 1900 is similar to the acoustic wavedevice 500 of FIGS. 5A-5E with some notable differences. The MPS of theacoustic wave device 1900 includes not only the carrier substrate 1902,the layer of dielectric material 1904, and the layer of piezoelectricmaterial 1906, but also an additional layer 1905 disposed between thecarrier substrate 1902 and the layer of dielectric material 1904. Theadditional layer 1905 may include or consist of, for example, aluminumnitride, silicon nitride, poly-silicon, or amorphous silicon. Theadditional layer 1905 may be a trap-rich layer that helps improve thequality factor Q of the acoustic wave device by reducing the effects ofparasitic surface conductivity on the upper surface of the carriersubstrate 1902.

Another difference between the acoustic wave device 1900 and acousticwave device 500 is that acoustic wave device 1900 includes short dummyelectrode fingers 1908D extending from sides of the mini-busbars 1912facing the central region C through a portion of the gap region G towardtips of IDT electrode fingers extending from the opposite busbars. Theshort dummy electrode fingers 1908D may have the same width and may bealigned with the IDT electrode fingers toward which they extend. Theshort dummy electrode fingers 1908D may increase the quality factor Q ofthe acoustic wave device 1900 by providing better confinement of theacoustic wave in the resonator while keeping transverse modessuppressed.

The acoustic wave device 1900 also includes portions of the IDTelectrode fingers 1908G within the gap region G between the mini-busbars1912 and main busbars that are thinner than the remainder of the IDTelectrode fingers in a direction of propagation of the main acousticwave through the device. These thinner portions 1908G of the IDTelectrode fingers may also increase the quality factor Q of the acousticwave device 1900 by providing better confinement of the acoustic wave inthe resonator.

The IDT electrode fingers of the acoustic wave device 1900 may besimilar to those of previously described embodiments in that theyinclude a lower layer 1908B of a high density material such as Mo or Wand an upper layer 1908A having a higher conductivity but lower densitythan the lower layer 1908B. The upper layer 1908A may include or consistof Al, for example. In contrast to the previously described embodiments,the width w_(L) of the lower layer 1908B of the IDT electrode fingersmay be greater than the width w_(U) of a base of the upper layer 1908Ain the direction of propagation of the main acoustic wave through thedevice. Further, the upper layer 1908A of the IDT electrodes may includetapered sides and a flat upper surface such that it is trapezoidal incross-section. A protective layer or passivation film 1918 formed fromone or more of Silicon nitride, silicon oxide (or silicon dioxide), orsilicon oxynitride may cover the IDT electrode fingers and otherwiseexposed portions of the upper surface of the piezoelectric layer 1906.The tapered shape of the upper layer 1908A of the IDT electrode fingersmay be beneficial to more easily provide for conformal coverage by theprotective layer 1918. The tapered shape and reduced base width of theupper layer 1908A of the IDT electrode fingers also gives less massloading than a square shaped electrode. Static capacitance of the IDTelectrode fingers is determined by the width of the lower layer 1908B.Too much mass loading may result in a reduction in quality factor Q ofthe device. By using IDT electrodes with tapered upper layers 1908A, ahigh Q may be maintained along with sufficient static capacitance.

Each of the above described embodiments suppress the transverse modesthrough a piston mode distribution, that is implemented without adecrease in the duty factor of the of the central region C of the IDT,preventing an unwanted increase in size of the device. Moreover, theformation of the trench portions through etching results in an easierfabrication, that is less likely to damage the central region C of theIDT. Lastly, the combination of the trench portions with the MPS resultsin a device with a high Q-factor, high electromechanical couplingcoefficient (K2), excellent temperature coefficient of frequency (TCF),and high power durability.

The embodiments of the acoustic wave device disclosed herein may be usedin various different implementations. The acoustic wave device may beused in any device that includes an IDT. For example, the acoustic wavedevice may be used in various types of acoustic wave resonators and/orfilters, including 1-port resonators, 2-port resonators, ladder filters,and the like. In a resonator configuration, one or more reflectorelectrodes may be included surrounding/sandwiching the IDT. Although theembodiments above have been described with only one IDT, otherconfigurations are possible, as would be understood by the skilledperson.

It should be appreciated that the various embodiments of acoustic wavedevices illustrated in the figures, as well as the other circuitelements illustrated in other figures presented herein, are illustratedin a highly simplified form. The relative dimensions of the differentfeatures are not shown to scale. Further, typical acoustic wave deviceswould commonly include a far greater number of electrode fingers in theIDTs than illustrated.

The concepts and embodiments of acoustic wave devices described hereinare applicable to various types of devices, as would be understood bythe skilled person. For example, the invention may be applied tofilters, duplexers, diplexers or the like. The suppression of transversemodes in the above described acoustic wave devices may lead to anoverall improvement in the overall functioning of the circuit.

For example, FIG. 20 shows an example of a SAW filter 2000 whichmultiple acoustic wave devices as disclosed herein may be combined. FIG.20 shows an RF ladder filter 2000 including a plurality of seriesresonators R1, R3, R5, R7, and R9, and a plurality of parallel (orshunt) resonators R2, R4, R6, and R8. As shown, the plurality of seriesresonators R1, R3, R5, R7, and R9 are connected in series between theinput and the output of the RF ladder filter, and the plurality ofparallel resonators R2, R4, R6, and R8 are respectively connectedbetween nodes between adjacent series resonators and ground in a shuntconfiguration. Other filter structures and other circuit structuresknown in the art that may include SAW devices or resonators, forexample, duplexers, baluns, etc., may also be formed including examplesof acoustic wave devices as disclosed herein.

Moreover, embodiments of acoustic wave devices discussed herein can beimplemented in a variety of packaged modules. Some examples of packagedmodules will now be discussed in which any suitable principles andadvantages of the acoustic wave devices discussed herein can beimplemented. FIGS. 21, 22, and 23 are schematic block diagrams ofillustrative packaged modules and devices according to certainembodiments.

As discussed above, acoustic wave devices, such as those of FIGS. 5, 10,12, 14, 17 and 18 , can be used in radio frequency (RF) filters. Inturn, an RF filter such as the SAW filter of FIG. 19 , may beincorporated into and packaged as a module that may ultimately be usedin an electronic device, such as a wireless communications device, forexample. FIG. 21 is a block diagram illustrating one example of a module2115 including a SAW filter 2100. The SAW filter 2100 may be implementedon one or more die(s) 2125 including one or more connection pads 2122.For example, the SAW filter 2100 may include a connection pad 2122 thatcorresponds to an input contact for the SAW filter and anotherconnection pad 2122 that corresponds to an output contact for the SAWfilter. The packaged module 2115 includes a packaging substrate 2130that is configured to receive a plurality of components, including thedie 2125. A plurality of connection pads 2132 can be disposed on thepackaging substrate 2130, and the various connection pads 2122 of theSAW filter die 2125 can be connected to the connection pads 2132 on thepackaging substrate 2130 via electrical connectors 2134, which can besolder bumps or wirebonds, for example, to allow for passing of varioussignals to and from the SAW filter 2100. The module 2115 may optionallyfurther include other circuitry die 2140, for example, one or moreadditional filter(s), amplifiers, pre-filters, modulators, demodulators,down converters, and the like, as would be known to one of skill in theart of semiconductor fabrication in view of the disclosure herein. Insome embodiments, the module 2115 can also include one or more packagingstructures to, for example, provide protection and facilitate easierhandling of the module 2115. Such a packaging structure can include anovermold formed over the packaging substrate 2130 and dimensioned tosubstantially encapsulate the various circuits and components thereon.

Various examples and embodiments of the SAW filter 2100 can be used in awide variety of electronic devices. For example, the SAW filter 2100 canbe used in an antenna duplexer, which itself can be incorporated into avariety of electronic devices, such as RF front-end modules andcommunication devices.

Referring to FIG. 22 , there is illustrated a block diagram of oneexample of a front-end module 2200, which may be used in an electronicdevice such as a wireless communications device (e.g., a mobile phone)for example. The front-end module 2200 includes an antenna duplexer 2210having a common node 2202, an input node 2204, and an output node 2206.An antenna 2310 is connected to the common node 2202.

The antenna duplexer 2210 may include one or more transmission filters2212 connected between the input node 2204 and the common node 2202, andone or more reception filters 2214 connected between the common node2202 and the output node 2206. The passband(s) of the transmissionfilter(s) are different from the passband(s) of the reception filters.Examples of the SAW filter 2100 can be used to form the transmissionfilter(s) 2212 and/or the reception filter(s) 2214. An inductor or othermatching component 2220 may be connected at the common node.

The front-end module 2200 further includes a transmitter circuit 2232connected to the input node 2204 of the duplexer 2210 and a receivercircuit 2234 connected to the output node 2206 of the duplexer 2210. Thetransmitter circuit 2232 can generate signals for transmission via theantenna 2310, and the receiver circuit 2234 can receive and processsignals received via the antenna 2310. In some embodiments, the receiverand transmitter circuits are implemented as separate components, asshown in FIG. 22 , however, in other embodiments these components may beintegrated into a common transceiver circuit or module. As will beappreciated by those skilled in the art, the front-end module 2200 mayinclude other components that are not illustrated in FIG. 22 including,but not limited to, switches, electromagnetic couplers, amplifiers,processors, and the like.

FIG. 23 is a block diagram of one example of a wireless device 2300including the antenna duplexer 2210 shown in FIG. 22 . The wirelessdevice 2300 can be a cellular phone, smart phone, tablet, modem,communication network or any other portable or non-portable deviceconfigured for voice or data communication. The wireless device 2300 canreceive and transmit signals from the antenna 2310. The wireless deviceincludes an embodiment of a front-end module 2200 similar to thatdiscussed above with reference to FIG. 22 . The front-end module 2200includes the duplexer 2210, as discussed above. In the example shown inFIG. 23 the front-end module 2200 further includes an antenna switch2240, which can be configured to switch between different frequencybands or modes, such as transmit and receive modes, for example. In theexample illustrated in FIG. 23 , the antenna switch 2240 is positionedbetween the duplexer 2210 and the antenna 2310; however, in otherexamples the duplexer 2210 can be positioned between the antenna switch2240 and the antenna 2310. In other examples the antenna switch 2240 andthe duplexer 2210 can be integrated into a single component.

The front-end module 2200 includes a transceiver 2230 that is configuredto generate signals for transmission or to process received signals. Thetransceiver 2230 can include the transmitter circuit 2232, which can beconnected to the input node 2204 of the duplexer 2210, and the receivercircuit 2234, which can be connected to the output node 2206 of theduplexer 2210, as shown in the example of FIG. 22 .

Signals generated for transmission by the transmitter circuit 2232 arereceived by a power amplifier (PA) module 2250, which amplifies thegenerated signals from the transceiver 2230. The power amplifier module2250 can include one or more power amplifiers. The power amplifiermodule 2250 can be used to amplify a wide variety of RF or otherfrequency-band transmission signals. For example, the power amplifiermodule 2250 can receive an enable signal that can be used to pulse theoutput of the power amplifier to aid in transmitting a wireless localarea network (WLAN) signal or any other suitable pulsed signal. Thepower amplifier module 2250 can be configured to amplify any of avariety of types of signal, including, for example, a Global System forMobile (GSM) signal, a code division multiple access (CDMA) signal, aW-CDMA signal, a Long-Term Evolution (LTE) signal, or an EDGE signal. Incertain embodiments, the power amplifier module 2250 and associatedcomponents including switches and the like can be fabricated on galliumarsenide (GaAs) substrates using, for example, high-electron mobilitytransistors (pHEMT) or insulated-gate bipolar transistors (BiFET), or ona silicon substrate using complementary metal-oxide semiconductor (CMOS)field effect transistors.

Still referring to FIG. 23 , the front-end module 2200 may furtherinclude a low noise amplifier (LNA) module 2260, which amplifiesreceived signals from the antenna 2310 and provides the amplifiedsignals to the receiver circuit 2234 of the transceiver 2230.

The wireless device 2300 of FIG. 23 further includes a power managementsub-system 2320 that is connected to the transceiver 2230 and managesthe power for the operation of the wireless device 2300. The powermanagement system 2320 can also control the operation of a basebandsub-system 2330 and various other components of the wireless device2300. The power management system 2320 can include, or can be connectedto, a battery (not shown) that supplies power for the various componentsof the wireless device 2300. The power management system 2320 canfurther include one or more processors or controllers that can controlthe transmission of signals, for example. In one embodiment, thebaseband sub-system 2330 is connected to a user interface 2340 tofacilitate various input and output of voice and/or data provided to andreceived from the user. The baseband sub-system 2330 can also beconnected to memory 2350 that is configured to store data and/orinstructions to facilitate the operation of the wireless device, and/orto provide storage of information for the user.

Any of the embodiments described above can be implemented in associationwith mobile devices such as cellular handsets. The principles andadvantages of the embodiments can be used for any systems or apparatus,such as any uplink wireless communication device, that could benefitfrom any of the embodiments described herein. The teachings herein areapplicable to a variety of systems. Although this disclosure includessome example embodiments, the teachings described herein can be appliedto a variety of structures. Any of the principles and advantagesdiscussed herein can be implemented in association with RF circuitsconfigured to process signals in a range from about 30 kHz to 5 GHz,such as in a range from about 500 MHz to 3 GHz.

Further examples of the electronic devices that aspects of thisdisclosure may be implemented include, but are not limited to, consumerelectronic products, parts of the consumer electronic products such aspackaged radio frequency modules, uplink wireless communication devices,wireless communication infrastructure, electronic test equipment, etc.Examples of the electronic devices can include, but are not limited to,a mobile phone such as a smart phone, a wearable computing device suchas a smart watch or an ear piece, a telephone, a television, a computermonitor, a computer, a modem, a hand-held computer, a laptop computer, atablet computer, a microwave, a refrigerator, a vehicular electronicssystem such as an automotive electronics system, a stereo system, adigital music player, a radio, a camera such as a digital camera, aportable memory chip, a washer, a dryer, a washer/dryer, a copier, afacsimile machine, a scanner, a multi-functional peripheral device, awrist watch, a clock, etc.

Having described above several aspects of at least one embodiment, it isto be appreciated various alterations, modifications, and improvementswill readily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be part of thisdisclosure and are intended to be within the scope of this disclosure.Accordingly, the foregoing description and drawings are by way ofexample only, and the scope of the disclosure should be determined fromproper construction of the appended claims, and their equivalents.

What is claimed is:
 1. An acoustic wave device, comprising: a layer ofpiezoelectric material; a pair of interdigital transducer electrodesdisposed on an upper surface of the layer of piezoelectric material,each interdigital transducer electrode including a bus bar and aplurality of electrode fingers extending from the bus bar through acentral region of the interdigital transducer electrode towards an edgeregion of the interdigital transducer electrode, each of the pluralityof electrode fingers having a width in a direction perpendicular to theextension of the electrode fingers that is smaller in the edge regionsthan in the central regions; and trench portions located in the uppersurface of the layer of piezoelectric material, the trench portionsoverlapping with the edge regions of the interdigital transducerelectrodes.
 2. The acoustic wave device of claim 1 wherein the widths ofthe plurality of electrode fingers of each interdigital transducerelectrode are smaller in both the edge region of that interdigitaltransducer electrode and the edge region of the other interdigitaltransducer electrode.
 3. The acoustic wave device of claim 1 wherein aduty factor of the pair of interdigital transducer electrodes in theedge regions of the interdigital transducer electrodes is less than aduty factor of the pair of interdigital transducer electrodes in thecentral regions.
 4. The acoustic wave device of claim 1 wherein theelectrode fingers of each interdigital transducer electrode interleavewith one another in an active region of the pair of interdigitaltransducer electrodes, and form gap regions between the ends of thefingers of one of the interdigital transducer electrodes and the bus barof the other interdigital transducer electrode, and the edge regions ofthe pair of interdigital transducer electrodes are located within theactive region and on opposing sides of the active region at the distalends of the electrode fingers.
 5. The acoustic wave device of claim 4wherein the active region includes the central regions and the edgeregions of the interdigital transducer electrodes, each edge regionextending from the tips of the plurality of electrode fingers of one ofthe interdigital transducer electrodes towards the center of the centralregion.
 6. The acoustic wave device of claim 4 wherein the trenchportions in the upper surface of the layer of piezoelectric materialalso overlap with at least part of the gap regions.
 7. The acoustic wavedevice of claim 4 wherein each of the interdigital transducer electrodesincludes a second bus bar that is located within the gap region.
 8. Theacoustic wave device of claim 7 wherein the plurality of electrodefingers have lesser widths in regions between the bus bar and second busbar of each interdigital transducer electrode.
 9. The acoustic wavedevice of claim 1 wherein the trench portions are located in the areasof the upper surface of the layer of piezoelectric material that areoverlapped by the edge regions of the interdigital transducer electrodesand are not covered by the interdigital transducer electrodes.
 10. Theacoustic wave device of claim 1 wherein the trench portions extenddiscontinuously in a direction of propagation of an acoustic wave to begenerated by the pair of interdigital transducer electrodes.
 11. Theacoustic wave device of claim 1 further comprising a layer of dielectricmaterial including one of silicon dioxide or doped silicon, the layerdielectric material having an upper surface disposed against a lowersurface of the layer of piezoelectric material.
 12. The acoustic wavedevice of claim 11 further comprising a carrier substrate, the carriersubstrate including a second layer of one of aluminum nitride, siliconnitride, polysilicon, or amorphous silicon an upper surface disposedagainst a lower surface of the layer of dielectric material and a thirdlayer of one of silicon, silicon carbide, sapphire, diamond-like carbonor quartz having an upper surface disposed against a layer surface ofthe second layer.
 13. The acoustic wave device of claim 1 wherein eachinterdigital transducer electrode is formed from a single layer of etchresistant material selected from the group consisting of copper,platinum, tungsten, molybdenum, ruthenium, iridium, gold, and silver.14. The acoustic wave device of claim 1 wherein each interdigitaltransducer electrode is formed from one or more lower layers of materialand an upper layer of etch resistant material selected from the groupconsisting of copper, platinum, tungsten, molybdenum, ruthenium,iridium, gold, and silver.
 15. The acoustic wave device of claim 1wherein each interdigital transducer electrode includes a mask layer onthe upper surface of the interdigital transducer electrode.
 16. Theacoustic wave device of claim 15 wherein the mask layer is a layer ofchromium.
 17. The acoustic wave device of claim 1 further comprising aprotective layer disposed over the upper surfaces of the pair ofinterdigital transducer electrodes and the layer of piezoelectricmaterial.
 18. The acoustic wave device of claim 17 wherein theprotective layer is formed from one or more of the group consisting ofsilicon nitride, silicon oxynitride, and silicon dioxide.
 19. A radiofrequency filter comprising at least one acoustic wave device, theacoustic wave device including: a layer of piezoelectric material; apair of interdigital transducer electrodes disposed on an upper surfaceof the layer of piezoelectric material, each interdigital transducerelectrode including a bus bar and a plurality of electrode fingersextending from the bus bar through a central region of the interdigitaltransducer electrode towards an edge region of the interdigitaltransducer electrode, each of the plurality of electrode fingers havinga width in a direction perpendicular to the extension of the electrodefingers that is smaller in the edge regions than in the central regions;and trench portions located in the upper surface of the layer ofpiezoelectric material, the trench portions overlapping with the edgeregions of the interdigital transducer electrodes.
 20. An electronicsmodule comprising at least one radio frequency filter that includes atleast one acoustic wave device, the at least one acoustic wave deviceincluding: a layer of piezoelectric material; a pair of interdigitaltransducer electrodes disposed on an upper surface of the layer ofpiezoelectric material, each interdigital transducer electrode includinga bus bar and a plurality of electrode fingers extending from the busbar through a central region of the interdigital transducer electrodetowards an edge region of the interdigital transducer electrode, each ofthe plurality of electrode fingers having a width in a directionperpendicular to the extension of the electrode fingers that is smallerin the edge regions than in the central regions; and trench portionslocated in the upper surface of the layer of piezoelectric material, thetrench portions overlapping with the edge regions of the interdigitaltransducer electrodes.